1. Field of the Invention
The present invention relates to high-frequency filters, complex electronic components using the filters, and portable radio apparatuses using the filters or the components, and more particularly, to a high-frequency filter formed of a plurality of transmission lines side-coupled in a plurality of stages, a complex electronic component using the filter, and a portable radio apparatus using the filter or the electronic component.
2. Description of the Related Art
In recent years, compact, high-performance portable radio apparatuses have been increasingly developed in the mobile communication field. To this end, a high-frequency circuit needs to have higher performance. Among high-frequency circuits, a high-frequency filter, which has a great effect on the performance of a portable radio apparatus, is strongly required to be made compact and to have higher performance. A conventional side-coupled high-frequency filter using a transmission line will be described below. FIG. 10 is a block diagram of a conventional side-coupled high-frequency filter. A high-frequency filter 50 serving as a bandpass filter has three transmission lines SL51 to SL53 which are side-coupled in three stages. The transmission lines SL51 to SL53 are connected in parallel to capacitors C51 to C53 for parallel resonance, respectively. One end of the transmission line SL51 is connected to an input terminal Pi through an input capacitor C54. One end of the transmission line SL53 is connected to an output terminal Po through an output capacitor C55. The other ends of the transmission lines SL51 and SL53 and one end of the transmission line SL52 are connected to the ground.
In the above conventional high-frequency filter, however, as shown in FIG. 11, the frequency characteristic is gentle at both sides of the center frequency, no pole exists at both sides of the center frequency, and attenuation is insufficient in the vicinity of the center frequency.